The use of composite structures in lieu of aluminum for commercial aircraft offers potential weight savings and performance improvements. One application where the use of composites is preferred is engine nacelle acoustical linings. The perforated structure of such linings helps attenuate engine noise.
Currently, nacelle acoustical linings for Boeing.TM. commercial aircraft are based on aluminum sandwich construction. The linings consist of an aluminum face sheet bonded to an aluminum honeycomb core and backed with a bonded, load carrying perforated inner skin. Other aircraft manufacturers make perforated backings by patching several perforated laminated composite sections together. This option has been unattractive to the Boeing Company in the past because of its nacelle load carrying requirements and the difficulty of making very large acoustical linings with complex contours.
Methods of making large, perforated, composite panels by conventional vacuum bagging and perforation techniques were considered. These included impregnating open hold fabric with resin, using a large vacuum press with either hard or soft tooling, filament winding and resin impregnation, stamping lay-ups and resin transfer molding. All of these methods require expensive tooling. Moreover, because of the large size and complex contours of nacelle linings, the tooling for each of these methods was very expensive. It is also necessary to lay-up each sheet only one layer at time and then debulk preparatory to piercing and final cure. This is time consuming and, therefore, expensive.
Also considered were various ways of forming perforations in preformed sheets such as drilling, abrasion, stamping, water jet, laser beam and electron beams. Each of these methods causes damage to reinforcing fibers weakening the panel. To compensate for such weakening, thicker, heavier cross-sections would be required.
A method of making non-structural perforated fiberglass/epoxy skins is taught in U.S. Pat. No. 3,787,546 to Pratt et al. The method entails placing a partially cured plastic fiberglass reinforced sheet over injection molded plastic studs on a pin mandrel. A pressure transmitting blanket is laid over the sheet and the assembly is vacuum bagged. The studs push through the sheet during autoclaving without damaging the glass fibers of the reinforced sheet.
The process of the '546 patent is limited in ways which make it inappropriate for making large, contoured, multilayered structural laminates. For example, the pin mandrel and ply lay-up must be formed to contour. Any attempt to form a compound results in buckling of the pin mandrel. Where small pieces are joined together to make a larger piece, this is accommodated by manufacturing perforated sheets with excess size and cutting away buckled regions. The size of perforated sheets that can be made is limited by the size of molded pin mandrel which can be obtained and by the size of press available for perforating plies. If the press pressure is too great, the pins on the mandrel are squashed, do not perforate effectively, and are difficult to remove from the autoclaved sheet.
Accordingly, this invention overcomes the shortcomings of Prior art methods by providing a method and means for making very large, perforated, fiber-reinforced, multilayer laminates in an efficient and cost-effective manner.